Thermal sensors are utilized in automotive technology to an increasing extent. As discussed earlier, such thermal sensors may also be employed for side-impact sensing. Owing to their use in motor vehicles, the thermal sensors are exposed to greatly fluctuating weather influences. Given such weather conditions, it is possible, in particular, that the thermal sensor, which must be in contact with its environment, has a deposit of liquid, i.e., water. This may be the result of bedewing, icing or contamination of the sensor.
The present invention provides a thermal sensor which includes an arrangement for removal of such deposits. In further developments, the thermal sensor is equipped with an arrangement for detection of the deposits and, lastly, also includes a catalytic layer, which is provided on the sensing element of the thermal sensor so as to remove the deposit by heating.
A thermal sensor has a sensing element to detect the temperature and an evaluation circuit for an output signal of the sensing element. In general, the sensing element is realized by metal-film resistors on a thermally insulating membrane. The evaluation circuit senses the signals from the sensing element, which are amplified and digitized by the evaluation circuit. They may then be transmitted to a control unit. The arrangement for removing the deposit from the sensing element is typically realized by heating. This may be generated by heating of the sensing element itself or by indirect heating. The catalytic layer may further improve this deposit removal via heating.
FIG. 1 shows the thermal sensor in the form of a block diagram. A sensing element 1 is connected to an evaluation circuit 2. As described above, evaluation circuit 2 has a measuring amplifier, an analog-digital converter and signal processing. Signal processing is normally realized by a processor or an ASIC. A trigger circuit 3, which triggers sensing element 1 and is itself triggered by evaluation circuit 2, establishes a closed-loop control circuit. Sensing element 1 is able to be triggered here by trigger circuit 3 as a function of the evaluated signals. To this end, trigger circuit 3 includes current sources and possibly a power amplifier.
FIG. 2 shows a realization of the sensing element. Metal-film resistors RT and RH are situated on a membrane 4. FIG. 2 shows a plan view and a section through the center of the structure. Membrane 4 has been produced by etching of a substrate, preferably a semi-conducting substrate. Metal-film resistors RT and RH have been deposited on the membrane using deposition technologies such as vapor deposition or electrode deposition. Platinum is usually used as material for the metal film. Resistors RT and RH form a small meander structure and are electrically separated from one another. However, they are arranged close enough to each other to allow indirect heating. Resistor RT is used to measure the temperature increase during use as side-impact sensor. Using evaluation circuit 2, its resistance value is measured and a resistance-proportional signal UDT is generated. With the aid of second resistor RH, which is thermally coupled to first resistor RT, a temperature signal may be simulated. Trigger circuit 3, which triggers RH with the aid of a heating voltage UH or a heating current IH, is required for this purpose.
FIG. 3 shows the situation just described in the form of a simple substitute circuit diagram. Resistor RH is connected to trigger circuit 3 and is heated thereby via a current IH or a voltage UH. Voltage UDT, which is measured by evaluation circuit 2, is tapped above resistor RT.
FIG. 4 shows the signal characteristic of the signals just described in the form of a diagram. The upper diagram describes voltage UH. This is voltage UH, which is used to heat resistor RH, a sinusoidal voltage 10 being utilized in this case. In the lower voltage timing diagram in which the time is plotted on the abscissa and the voltage on the ordinate, voltage UDT is shown on the ordinate, the voltage being measured by evaluation circuit 2.
If resistor RH is triggered by the temporally variable signal UH 10, periodic heating and cooling of resistors RH and RT will result. This heating and cooling is measured with the aid of evaluation circuit 2, and a temperature-dependent output signal UDT is generated. For a known trigger signal, a signal with a defined amplitude and a defined phase shift will always come about with an uncontaminated and undamaged sensor element. This is indicated by reference numeral 20 in the lower diagram. If a sensor element has dewing, icing or contamination, a changed phase shift and/or a changed amplitude will result for output signal UDT due to the increased mass and heat dissipation of the resistors, so that signal 30 will be generated. A measurement of these parameters thus allows a deposit to be detected. In FIG. 4, the method is shown with a sinusoidal signal; however, other signal shapes, such as a rectangle or saw-tooth, may basically be used as well.
The measuring principle according to the present invention may also be applied to sensing elements in which heating resistor element RH has been omitted. This is shown in FIG. 5. Here, only resistor RT is present. During measurements, this resistor RT is used both for heating and for the measurement. To this end, resistor RT is first coupled to trigger circuit 3. After heating, the resistor is decoupled from trigger circuit 3 and connected to evaluation circuit 2. Signal UDT therefore represents the cooling process of the sensing element. The cooling process of a sensor element without deposits is of very brief duration, approximately 0.1 to 50 ms; in contrast, the cooling process of a sensing element with deposits is much slower since the thermally relevant mass is greater. If the time is measured for switching off the heating until resistor RT reaches a specific temperature, i.e., a voltage value UTDT, a time delay δT, which is able to be measured, results for a sensing element that includes a deposit.
FIG. 6 shows this characteristic in a voltage timing diagram. Two curves representing different decay times for the cooling of resistor RDT, are depicted here. The decay curve on the right shows the cooling of a contaminated resistor, i.e., the time is longer than that for a non-contaminated sensor. This characteristic is illustrated by the left curve, which exhibits a steeper drop, i.e., cooling. The switchover between evaluation circuit 2 and trigger circuit 3 may be implemented electronically or electromechanically.
Icing or dewing of the thermal sensor may be avoided or removed by heating the sensor. If resistors RT and RH are present, RH may be utilized for the heating. Heating may be performed with the aid of trigger circuit 3 without switching.
If only resistor RT is present, it must be decoupled from evaluation circuit 2 for the heating process and be coupled to trigger circuit 3.
Heating may be carried out slowly or in a pulse. In slow heating, the entire volume of the ice or water film will evaporate. In pulse-type heating, a film evaporation may be brought about which results in the ice or fluid layer sliding off.
By using a catalytic layer in the area of membrane 4, a pyrolytic cleaning of membrane 4 may be implemented at already relatively low temperatures. This makes it possible to remove organic deposits from the sensor. The temperature required for starting the pyrolytic cleaning may be generated by heating membrane 4 with the aid of resistors RT and RH.